fix: link verbosity to logging level

[kemingy]

Summary

• map BUB_VERBOSE values to Loguru levels: 0 → INFO, 1 → DEBUG, and 2 → TRACE
• configure both the stderr and optional Logfire sinks with the selected level
• load settings after BubFramework loads the YAML configuration, so file and environment settings are respected

Why

Loguru defaults new sinks to DEBUG, which made Bub print the optional Logfire import diagnostic during normal CLI startup. The existing verbose setting was not connected to logging.

Verification

• make check
• uv run pytest -q tests/test_cli_help.py tests/test_settings.py (14 passed)
• make test (330 passed, 1 skipped, 4 unrelated existing failures in tests/test_install_scripts.py because website/public/install.sh expands an unset EXTRA_DEPENDENCIES array under macOS /bin/bash)
• tox not run locally; the contribution guide notes that CI runs the Python version matrix